As a method for manufacturing a cylindrical part having a thin-walled portion, such as, for example, a cylindrical part 300 with a thin-walled portion 302 at its one end, as shown in FIG. 13, a cutting process is possible. It is, however, disadvantageous in that a manufacturing cost will be increased because a working time becomes longer if the thin-walled portion 302 is formed by the cutting process.
As another method for manufacturing the cylindrical part with the thin-walled portion 302, an extrusion process can be applied, as disclosed in the following publications:                Japanese Patent Publication No. H5-38546,        Japanese Patent Publication No. 2002-153940,        Japanese Patent Publication No. 2000-74040,        Japanese Patent Publication No. H9-103839,        Japanese Patent Publication No. H8-270670,        Japanese Patent Publication No. H7-144247,        Japanese Patent Publication No. 2000-71046, and        Japanese Patent Publication No. H7-275990.        
For example, as shown in FIGS. 14A and 14B, a base metal 340 of a cylindrical shape is held by a pilot 320 and a thin-walled portion 302 is formed by a backward extrusion process by a die 310 and a punch 330 having a step portion.
It is, however, necessary in such a method that a large pushing force must be applied to the punch 330 so that the punch 330 is plunged into the base metal 340, because the thin-walled portion 302 is formed in a manner that a part of the material for the base metal 340 flows backwardly (namely, in an opposite direction of the punch 330) being pressed. Furthermore, in such a backward extrusion process, the material of the base metal 340 flows, as indicated by an arrow in FIG. 14B, into a space formed between the die 340 and the punch 330, which is smaller than a space corresponding to a heavy-walled portion. Accordingly, a pressing force at the thin-walled portion, which is applied by the punch 330 to the base metal 340 in a radially outward direction toward the die 310, becomes extremely high. The pressing force becomes higher and higher, when thickness of the thin-walled portion becomes smaller and smaller.
As a result, a life time of working jigs, that is the die 310 and the punch 330, will become shorter, the base metal 340 may change its shape. Furthermore, the base metal 340 and the die 310, as well as the base metal 340 and the punch 330 may be adhered to each other, and therefore it may happen that the cylindrical part 300 can not be detached from the die 310. In FIGS. 14A and 14B, the material backwardly flows in an inner surface of the base metal, the same problem may occur in the case that a thin-walled portion is formed wherein a material backwardly flows in an outer surface of a base metal.
The inventors of the present invention studied another method for manufacturing a cylindrical part having a thin-walled portion at its one end, by an ironing process. In this manufacturing process, a base metal of a cylindrical shape with a closed end is formed, an inner wall of the closed end of the base metal is pushed by a punch, so that the base metal is extended by the ironing process during the punch is plunged into the base metal, a thin-walled portion is formed adjacent to the closed end, and the closed end is punched out, to finally form a cylindrical part having the thin-walled portion at one end. Since the base metal is extended in the ironing process, the ironing process can be performed by a smaller pressing force than that for the backward extrusion process. Accordingly, the adhesion between the base metal and the working jigs (the die and punch) may not occur. And since the base metal is processed by a smaller pressing force, the life time of the working jigs can be elongated.
In the above method for manufacturing the thin-walled portion adjacent to the closed end of the cylindrical base metal by the ironing process, however, it is not possible to form an inner side step portion at a boundary between the thin-walled portion and a heavy-walled portion. In the case that an outer side step portion is formed on a cylindrical part, an inner wall of a closed end of a base metal 400 is plunged by a punch 410, and an outer peripheral portion of the base metal 400 having the closed end is extended by the ironing process by a step portion 422 formed at an inner surface of a die 420. According to this method, the thin-walled portion 402 is formed adjacent to the closed end and the outer side step portion 404 due to a difference of thickness is formed on the outer periphery of the base metal 400.
In the above method, however, a material volume flowing from the thin-walled portion 402 to the heavy-walled portion 403 varies depending on a thickness of a base metal 400, when the base metals with a closed end has variation in its thickness before the base metal is processed, and thereby dimensional accuracy for a length of the heavy-walled portion is varied.